This invention is related to semiconductor laser having a double hetero-junction formed by semiconductor layers having different conductivities.
As is already well known, this type of semiconductor laser is produced by laminating an n-type semiconductor layer, an active layer which performs the laser operation and a p-type semiconductor in this order onto an n-type substrate to form a double hetero junction structure. A stripe structure is added to the above. Then, the carrier injection electrodes are formed on the back surface of the n-type substrate an upper part of the p-type semiconductor layer. With this, the recombination of carriers at the active layer is performed with a high efficiency and the commonly known laser operation is performed.
In a semiconductor laser such as described above, the lowering of the threshold current and the simplification of the oscillation mode are the main targets for development. However, the securing of high reliability is a problem which should be taken into account with the above development. Furthermore, the possibility of integration is a demand of the age.
As is also well known, the stripe structure constricts the drive current and improves the injection of the carriers in the specific area of the active layer. Thus, the carriers in the specific area recombine and limit the luminous area to a very narrow one. Thus, it becomes possible to achieve a lowering of the threshold current and simplification of the oscillation mode. Various stripe structures have been proposed.
For instance, there is a semicondoctor laser which an insulating layer is formed on the upper part of the p-type semiconductor layer leaving a narrow stripe section to inject carriers. In this case, the recombination area width becomes wider than that of the stripe since there is no current constriction in the section of the p-type semiconductor layer. In short, the simplification of the lateral mode is difficult. Also the heat radiation is poor because thermal conductivity of insulating layer is low value. Consequently, there is a resultant lowering of the reliability.
Another factor for lowering the reliability is indicated below. Namely, the aforementioned n-type semiconductor layer is formed by doping Te. Generally, an n-type semiconductor layer formed by doping Te has a poor crystallization and consenquently, the crystallization of the active layer to be formed on the n-type semiconductor layer is also poor. As a result, the threshold current is difficult to decrease and deterioration is accelerated. The last, with the conventional semiconductor lasers of this type, the n-type semiconductor layer is formed by lamination on an n-type substrate. The laser element of the double hetero-junction structure and the substrate are not electrically separated. Consequently, it is difficult to integrate the laser element together with other semiconductor elements, and it is also difficult to integrate the many laser elements on the same substrate.